Method for seismic surveying



United States Patent M 3,280,936 METHOD FOR SEISMIC SURVEYING WilliamHarry Mayne, San Antonio, Tex., assiguor to Petty GeophysicalEngineering Company, a corpora tion of Delaware Filed Mar. 7, 1961, Ser.No. 93,963 5 Claims. (Cl. 181-.5)

This invention relates to an improved method for conducting geophysicalstudies of the type wherein an arti ficial shock is imparted to theearth, and the seismic waves thereby propagated, are received afterreflection from various strata interfaces, or subsurface beds, by aplurality of seisrn-ometers, the received seismic energy being suitablyamplified, combined, and recorded. The principal object of the inventionis the provision of a method whereby the energy reflected from apreselected subsurface bed may be emphasized, and the undesired energyreflected from the other subsurface beds may be reduced to a minimum inthe final record.

This invention is an improvement of the method described in my US.Patent 2,732,906, issued January 31, 1956. In that patent, a method ofsurveying is disclosed in which provision is made for reducing theundesired or random energy in the final record. In the system disclosedin this patent, however, no provision is made for reducing the signalsresulting from higher order reflections from one of the strata. Asemployed herein, the term higher order reflections refers to multiplereflections between a given strata and the earths surface. For example,a second order reflection is one which travels from the earths surfaceto the strata, is reflected to the earths surface, is again reflecteddown to the strata and is reflected from the strata a second time to theearths surface. It is a well-established fact that in reflectionseismograph surveying, higher order reflections between a relativelyshallow, good reflecting strata and the surface may interfere with orobscure the reflections from deeper strata. It is also well known thatthe polarity (or phase) of these higher order reflections will alternatewith the order of the multiple of the reflection while the general formor character of the reflections remains unchanged. By this is meant thata second order reflection will have a polarity opposite to that of athird order reflection, but of the same polarity as a fourth orderreflection.

Accordingly, it is an object of this invention to provide an improvedmethod for seismic surveying.

It is another object of this invention to provide a method for seismicsurveying which makes possible the production of a recording, augmentingthe first order reflections and cancelling the higher order reflections.

It is a still further object of this invention to provide a method forseismic surveying in which successive orders of reflections above theprimary, or first, order are combined in pairs to cancel each other,while the first order reflection signals are combined with each other ofthe same polarity and phase to reinforce each other and further simplifythe process of distinguishing between the recorded signals.

It is a common practice by those skilled in geophysics to measuretypical velocities in a given area by lowering a geophone into a drilledhole, shoot near the surface and 3,280,936 Patented Oct. 25, 1966selected shotpoint S to a nearby similarly arbitrarily lo catedtransducer T along with the other information. Consequently the depth Dcan be easily determined. This depth can vary, of course, over aprospect, but a running check can be made as the survey progresses, andthe distances modified to compensate for these changes. Determination ofthe depth D of the second, or 1 interface is somewhat less straightforward but no less rigorous. From geological logs and the velocitysurvey previously mentioned, an interface (or change in formation) ischosen which would be most likely to yield a significant reflection. Theexistence of such a reflection can be confirmed by close study of therecords and thus determine its approximate arrival. Even though themultiples possess a greater amplitude than the primary reflection from Ifragments of the I reflection can usually be identified with areasonable degree of certainty, yet will not be of mapping quality.Thus, again using the known velocities, one skilled in the art canreasonably postulate the depth of the I interface, D Give-n thisinformation it is then a simple exercise in geometry to determine therequired distances. The general formulae for this determination is asfollows:

Let

V zVelocity from surface to I interface V =Velocity from I interface to1 interface D =Depth to 1, interface D =Distance between I and Iinterfaces X =Horizontal distance from midpoint to T or S the transducerand shotpoint.

Hence, using shotpoint S and receiving point T the travel time for the Ireflection will be:

and the record time for the third order I reflection will Forinterference to occur, then, these times will be equal,

2 MCW i and record time for second order I reflection will be:

Thus:

raw-We I) which simplifies to:

in which X is the only unknown in a specific case. This is a fourthorder equation, and the most convenient solution will be a graphicalone.

and these times must also be equal.

These and various other objects and features of the invention will bemore clearly understood from a reading of the detailed description ofthe invention in conjunction with the drawing, in which:

The figure is a drawing in section showing a portion of the earthssurface and the locations of respective shot devices and transducers.

Referring now to the figure, there is depicted in section, a portion ofthe earths crust in which E represents the earths surface. 1, representsa first substrata, and I represents a second substrata. For the purposeof illustrating the principles of this invention, two shot producingdevices S and S are employed and a pair of transducers T and T arepositioned on the earths surface at predetermined distances from eachother. For example, the distance between the mid-point and the shotproducing device S may have been determined by the above method to be990 feet; the distance between the midpoint and transducer T will alsobe 990 feet.

Assume that it has been determined by the above method that the traveltime to the first interface 1 and the return to the earths surface E is.20 second. For the essentially vertical path from shot location S tothe transducer T the third order reflection between the surface and theinterface I would have a travel time of .60 second and would interferewith the primary reflection from the second interface I it beingexperimentally determined that the travel time to the second interfaceand return to the surface E is also .60 second. By placing shot locationS and transducer T 990 feet on opposite sides of S and T the secondorder reflection from interface I would have a travel time of .616second. The first order reflection from interface I would likewise havea travel time of .616 second. Thus if the independent recordings made atT and T were combined, the signal received at T would have to beadvanced in time by .016 second to obtain coincidence between the Ireflections. Since this advance would also advance the second orderreflection from I by the same amount, this second order I reflectionwould then coincide in time with the third order I reflection recordedat T but would be inverted in phase because of the difference in order.Thus the two interfering multiple reflections from I would cancel eachother while the two I reflections being of the same order wouldreinforce each other. Although only two pairs of paths are shown, otherpairs could be similarly chosen to obtain even better averagingconditions and the example illustrated is intended to be merely for thepurpose of explaining the basic principle involved.

While only first, second, and third order reflections are shown, it isunderstood that numerous additional orders of reflections take place,and these have been illustrated only for the purpose of explanation. Aswill be subsequently explained, a number of transducers are connected toa recording cable and individual records made for each of thetransducers. The independent recordings may then be combined in themanner suggested in my previously mentioned U.S. patent. The twointerfering multiple reflections from subsurface strata I will be ofadjacent order, namely, second and third order, and will cancel eachother when combined on a transfer magnetic tape since they will be ofopposite phase. The reflections from I will be combined on the transfermagnetic tape as signals which reinforce each other. It is to be notedthat elimination of a particular multiple reflect-ion will clarify aportion of the record at least equal to the vertical travel time of theI reflection. In the specific example cited this will result in a timewindow of .2 second in the zone where arrival of the significant 1reflection is expected, so that minor variations in its actual arrivaltime will be unimportant. Mapping of the I interface can thus proceed inorderly fashion with periodic adjustments in the distances as the trendof the depths change.

What is claimed is:

1. In a method of seismic surveying, the steps comprising observing thevelocity of sound travel from the earths surface to the first interface,observing the velocity of sound travel between the first and secondinterfaces, calculating the depth of the first interface, calculatingthe distance between the first and second interfaces, inserting thesevalues in Equation 8 appearing in the specification, graphicallyplotting said equation to determine the various solutions of saidequation, locating on the earths surface a first shot producing deviceand a first transducer at equal distances on opposite sides of apredetermined midpoint, locating a second shot producing device and itscompanion second transducer at different equal distances on oppositesides of said midpoint in accordance with one of the solutions of saidequation, firing the first shot pro ducing device and recording thesignal received by each of the transducers, firing the second shotproducing device and recording the signal received at each of thetransducers and algebraically re-recording the signals received atindividual ones of the transducers whereby second and higher ordersignal reflections are cancelled.

2. In a method of seismic surveying according to claim 1, furthercomprising the steps of moving said shot producing devices and saidtransducers a predetermined equal distance on a straight line along theearths surface to a new position and repeating each of the steps setforth in claim 1.

3. A method of seismic surveying comprising the steps of graphical-1yplotting Equation 8 appearing in the specification to determine thevarious solutions of said equation, locating on the earths surface afirst shot producing device and a first transducer at equal distances onopposite sides of a predetermined midpoint, locating a second shotproducing device and a second transducer on opposite sides of saidmidpoint, said last mentioned distances being equal to one of thesolutions of said equation, firing the first shot producing device andrecording the signal received by each of said transducers, firing thesecond shot producing device and recording the signal received at eachof the transducers and algebraically re-recording the signals receivedat individual ones of the transducers whereby second and higher ordersignal reflections are cancelled.

4. The method of seismic surveying according to claim 3 furthercomprising the steps of spacing transducers and shot producing devicesat equal distances on opposite sides of a said predetermined midpointwhich last mentioned distances are individually equal to respective onesof the solutions of said equation and repeating the steps of firing theshot producing devices, recording the signal received by respective onesof the transducers and re-recording the signals received at individualones of the trans ducers in a manner to cancel signals of opposite phasewhereby second and higher order recorded signal reflections arecancelled.

5. In a method of seismic surveying, the steps of experimentallydetermining velocities of sound travel from the earths surface to eachof two interfaces D and D having depths in that order to compute thedepth to the interfaces from observed travel time, using these resultsto determine the placement of the first shot and its companiontransducer in locations on the earth surface at equal horizontaldistances on opposite sides of a predetermined midpoint, and similarlylocating placement of a second shot and its companion transducer locatedat equal but different horizontal distances on opposite sides of saidmidpoint said second distance being selected to satisfy Equation 8,appearing in the specification, firing the first shot producing deviceand recording the signal received at the first transducer, firing thesecond shot producing device and recording a signal received at thesecond trans ducer with signals which may be algebraically combined,whereby a higher order signal reflection from the interface at depth Drecorded from the first shot will be retained by the next lower order ofsignal reflection from the same 5 6 interfiace recorded from the secondshot whenever the sig- 2,849,076 8/ 1958 Kaufman 181-.5 nal reflectionsfrom an interfiace at depth D recorded 2,906,363 9/1959 Clay 181-.5 fromthe two shots are adjusted for time coincidence. 2,970,876 2/ 1961 Moody181.5 2,993,555 7/1961 Wolf 181.5 References Cited by the Examiner 53,016,970 1 /1962 All et 181 5 UNITED STATES PA S 3,040,833 6/ 1962Mendenhlall et a1 181.5 3,105,568 10/1963 Jolly 181.5 1,799,398 4/1931Tay lor 181.5 2,148,422 2/1 9 Bl BENJAMIN A. BORCHELT, Primary Examiner.

$31? sparks 181 '5 10 CHARLES W. ROBINSON, CHESTER L. JUSTUS, miner181-5 SAMUEL FEINBERG E 2,260,217 10/1941 Eck'hardt et a1 181.52,336,053 12/1943 Athy et a1. 181.5 A. S. ALPERT, J. W. MILLS, M. F.HUBLER,

2,623,113 12/ 1952 Baylie et a1. 181-.5 Assistant Examiners.

1. IN A METHOD OF SEISMIC SURVEYING, THE STEPS COMPRISING OBSERVING THEVELOCITY OF SOUND TRAVEL FROM THE EARTH''S SURFACE TO THE FIRSTINTERFACE, OBSERVING THE VELOCITY OF SOUND TRAVEL BETWEEN THE FIRST ANDSECOND INTERFACES, CALCULATING THE DEPTH OF THE FIRST INTERFACES,CALCULATING THE DISTANCE BETWEEN THE FIRST AND SECOND INTERFACES,INSERTING THESE VALUE IN EQUATION 8 APPEARING IN THE SPECIFICATION,GRAPHICALLY PLOTTING SAID EQUATION, TO DETERMINE THE VARIOUS SOLUTIONSOF SAID EQUATION, LOCATING ON THE EARTH''S SURFACE A FIRST SHOTPRODUCING DEVICE AND A FIRST TRANSDUCER AT EQUAL DISTANCES ON OPPOSITESIDES OF A PREDETERMINED MIDPOINT, LOCATING A SECOND SHOT PRODUCINGDEVICE AND ITS COMPANION SECOND TRANSDUCER AT DIFFERENT EQUAL DISTANCESON OPPOSITE SIDES OF SAID MIDPOINT IN ACCORDANCE WITH ONE OF THESOLUTIONS OF SAID EQUATION, FIRING THE FIRST SHOT PRODUCING DEVICE ANDRECORDING THE SIGNAL RECEIVED BY EACH OF THE TRASDUCERS, FIRING THESECOND SHOT PRODUCING DEVICE AND RECORDING THE SIGNAL RECEIVED AT EACHOF THE TRANSDUCERS AND ALGEBRACIALLY RE-RECORDING THE SIGNALS RECEIVEDAT INDIVIDUAL ONES OF THE TRANSDUCERS WHEREBY SECOND AND HIGHER ORDERSIGNAL REFLECTIONS ARE CANCELLED.